Semiconductor power device having a top-side drain using a sinker trench

ABSTRACT

A semiconductor power device includes a plurality of groups of stripe-shaped trenches extending in a silicon region over a substrate, and a contiguous sinker trench completely surrounding each group of the plurality of stripe-shaped trenches so as to isolate the plurality of groups of stripe-shaped trenches from one another. The contiguous sinker trench extends from a top surface of the silicon region through the silicon region and terminates within the substrate. The contiguous sinker trench is lined with an insulator only along the sinker trench sidewalls so that a conductive material filling the contiguous sinker trench makes electrical contact with the substrate along the bottom of the contiguous sinker trench and makes electrical contact with an interconnect layer along the top of the contiguous sinker trench.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/038,184, filed Feb. 27, 2008, which is a continuation of U.S.application Ser. No. 11/194,060, filed Jul. 28, 2005, now U.S. Pat. No.7,352,036, which claims the benefit of U.S. Provisional Application No.60/598,678, filed Aug. 3, 2004. All are incorporated herein by referencein their entirety for all purposes. Also, this application relates toapplication Ser. No. 11/026,276 titled “Power Semiconductor Devices andMethods of Manufacture” filed Dec. 29, 2004, which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

This invention relates in general to semiconductor power devices andmore particularly to power devices with top-side drain contact using asinker trench.

Unlike integrated circuits (ICs) which have a lateral structure with allinterconnects available on the upper die surface, many powersemiconductor devices have a vertical structure with the back of the diebeing an active electrical connection. For example, in vertical powerMOSFET structures, the source and gate connections are at the topsurface of the die and the drain connection is on the back side of thedie. For some applications, it is desirable to make the drain connectionaccessible at the top side. Sinker trench structures are used for thispurpose.

In a first technique, diffusion sinkers extending from the top-side ofthe die down to the substrate (which forms the drain contact region ofthe device) are used to make the drain contact available at the topsurface of the die. A drawback of this technique is that the lateraldiffusion during the formation of the diffusion sinkers results inconsumption of a significant amount of the silicon area.

In a second technique, metal-filled vias extending from the top-side ofthe die clear through to the backside of the die are used to bring theback-side contact to the top-side of the die. Although, this techniquedoes not suffer from the loss of active area as in the diffusion sinkertechnique, it however requires formation of very deep vias which adds tothe complexity of the manufacturing process. Further, during conduction,the current is required to travel through long stretches of thesubstrate before reaching the drain contact, thus resulting in higherdevice on resistance Ron.

Thus, an improved trench structure for making a back-side contactavailable at the top-side is desirable.

BRIEF SUMMARY OF THE INVENTION

In accordance with an embodiment of the invention, a semiconductor powerdevice includes a substrate of a first conductivity type and anepitaxial layer of the first conductivity type over and in contact withthe substrate. A first trench extends into and terminates within theepitaxial layer. A sinker trench extends from the top surface of theepitaxial layer through the epitaxial layer and terminates within thesubstrate. The sinker trench is laterally spaced from the first trench,and is wider and extends deeper than the first trench. The sinker trenchis lined with an insulator only along the sinker trench sidewalls sothat a conductive material filling the sinker trench makes electricalcontact with the substrate along the bottom of the trench and makeselectrical contact with an interconnect layer along the top of thetrench.

In accordance with another embodiment of the invention, a semiconductorpower device is formed as follows. An epitaxial layer is formed over andin contact with a substrate. The epitaxial layer and the substrate areof a first conductivity type. A first opening for forming a first trenchand a second opening for forming a sinker trench are defined such thatthe second opening is wider than the first opening. A silicon etch isperformed to simultaneously etch through the first and second openingsto form the first trench and the sinker trench such that the firsttrench terminates within the epitaxial layer and the sinker trenchterminates within the substrate. The sinker trench sidewalls and bottomare lined with an insulator. The sinker trench is filled with aconductive material such that the conductive material makes electricalcontact with the substrate along the bottom of the sinker trench. Aninterconnect layer is formed over the epitaxial layer such that theinterconnect layer makes electrical contact with the conductive materialalong the top surface of the sinker trench.

In accordance with yet another embodiment of the invention, asemiconductor power device includes a plurality of groups ofstripe-shaped trenches extending in a silicon region over a substrate. Acontiguous sinker trench completely surrounds each group of theplurality of stripe-shaped trenches so as to isolate the plurality ofgroups of stripe-shaped trenches from one another. The contiguous sinkertrench extends from a top surface of the silicon region through thesilicon region and terminates within the substrate. The contiguoussinker trench is lined with an insulator only along the sinker trenchsidewalls so that a conductive material filling the contiguous sinkertrench makes electrical contact with the substrate along the bottom ofthe contiguous sinker trench and makes electrical contact with aninterconnect layer along the top of the contiguous sinker trench.

In accordance with yet another embodiment of the invention, asemiconductor power device includes a plurality of groups ofstripe-shaped gate trenches extending in a silicon region over asubstrate. Each of a plurality of stripe-shaped sinker trenches extendsbetween two adjacent groups of the plurality of groups of stripe-shapedgate trenches. The plurality of stripe-shaped sinker trenches extendfrom a top surface of the silicon region through the silicon region andterminate within the substrate. The plurality of stripe-shaped sinkertrenches are lined with an insulator only along the sinker trenchsidewalls so that a conductive material filling each sinker trench makeselectrical contact with the substrate along the bottom of the sinkertrench and makes electrical contact with an interconnect layer along thetop of the sinker trench.

In accordance with another embodiment of the invention, a semiconductorpackage device houses a die which includes a power device. The dieincludes a silicon region over a substrate. Each of a first plurality oftrenches extends in the silicon region. A contiguous sinker trenchextends along the perimeter of the die so as to completely surround thefirst plurality of trenches. The contiguous sinker trench extends from atop surface of the die through the silicon region and terminates withinthe substrate. The contiguous sinker trench is lined with an insulatoronly along the sinker trench sidewalls so that a conductive materialfilling the contiguous sinker trench makes electrical contact with thesubstrate along the bottom of the contiguous sinker trench and makeselectrical contact with an interconnect layer along the top of thecontiguous sinker trench. A plurality of interconnect balls arranged ina grid array includes an outer group of the plurality of interconnectballs electrically connecting to the conductive material in thecontiguous sinker trench.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified cross sectional view of an exemplary verticalpower device in accordance with an embodiment of the invention;

FIGS. 2-4 show various top layout views of a vertical power device withone or more sinker trenches in accordance with exemplary embodiments ofthe invention; and

FIG. 5 is a top view illustrating the locations of interconnect balls ina ball-grid array package relative to a sinker trench extending alongthe perimeter of a die housed in the ball-grid array package, inaccordance with an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with an embodiment of the present invention, a sinkertrench terminating within the silicon substrate is filled with a highlyconductive material such as doped polysilicon or metallic material. Thesinker trench is laterally spaced a predetermined distance from theactive region wherein gate trenches are formed. The sinker trench iswider and extends deeper than the gate trenches, and is lined with aninsulator only along its sidewalls. This technique eliminates the arealoss due to side diffusion of the diffusion sinker approach, and resultsin improved on-resistance since a more conductive material is used thandiffusion. Also, this technique requires a far shallower trench thanthat needed in the technique where a metal-filled trench extends fromthe top to the bottom of the die. The on-resistance is improved sincethe current need not travel through the entire depth of the substrate toreach the drain contact.

FIG. 1 shows a simplified cross sectional view of a verticaltrenched-gate power MOSFET structure 100 in accordance with an exemplaryembodiment of the invention. An n-type epitaxial layer 104 extends overan n-type substrate 102 which forms the back side drain. A sinker trench106 extends from the top surface of epitaxial layer 104 throughepitaxial layer 104 terminating within substrate 102. A dielectric layer110 lines the sinker trench sidewalls. Dielectric layer 110 may be fromany one of oxide, silicon nitride, silicon oxynitride, multilayer ofoxide and nitride, any known low k insulating material, and any knownhigh k insulating material. “Oxide” as used in this disclosure means achemical vapor deposited oxide (Si_(x)O_(y)) or a thermally grownsilicon dioxide (SiO₂). Sinker trench 106 is filled with a conductivematerial 108 such as doped polysilicon, selective epitaxial silicon(SEG), metal, or metallic compounds. Conductive material 108 is inelectrical contact with substrate 102 along the bottom of sinker trench106. Conductive material 108 thus makes the back-side drain availablealong the top side for interconnection. With the drain contact moved tothe top surface, a back-side metal for contacting substrate 102 is nolonger needed, but could be used in conjunction with the top sidecontact. The back side metal layer may be included for other purposessuch as preventing the die from cracking and improving the heat transferproperties of the device.

Well regions 114 of p-type conductivity extend along an upper portion ofepitaxial layer 104. Gate trenches 112 are laterally spaced from sinkertrench 106 by a predetermined distance S1, and vertically extend fromthe top surface through p-type well regions 114 terminating at apredetermined depth within epitaxial layer 104. Sinker trench 106 iswider and deeper than gate trenches 112. Gate trenches 112 are linedwith a dielectric layer 116. The dielectric along the bottom of gatetrenches 112 may optionally be made thicker than the dielectric alongthe gate trench sidewalls. Each gate trench 112 includes a gateelectrode 118 and a dielectric layer 120 atop gate electrode 118 toreduce the gate to drain capacitance. Source regions 122 of n-typeconductivity extend along an upper portion of well regions 114. Sourceregions 122 overlap gate electrodes 118 along the vertical dimension. Ascan be seen well region 114 terminates a distance away from sinkertrench 106. In one embodiment, this distance is dictated by the deviceblocking voltage rating. In another embodiment, well region 114terminates at and thus abuts sinker trench 106. In this embodiment, forhigher blocking voltage ratings, the thickness of the dielectric layeralong sinker trench sidewalls needs to be made larger since the sinkerdielectric is required to withstand a higher voltage. This may require awider sinker trench if the conductive material 108 is required to have aminimum width for current handling purposes.

In the on state, a conduction channel from source regions 122 toepitaxial layer 104 is formed in well regions 114 along gate trenchsidewalls. A current thus flows from drain terminal 124 verticallythrough conductive material 108 of sinker trench 106, then laterallythrough substrate 102, and finally vertically through epitaxial layer104, the conduction channel in well regions 114, and source regions 122,to source terminal 126.

While the width of the gate trenches is generally kept as small as themanufacturing technology allows to maximize the packing density, a widersinker trench is generally more desirable. A wider sinker trench iseasier to fill, has lower resistance, and can more easily be extendeddeeper if needed. In one embodiment, sinker trench 106 and gate trenches114 are formed at the same time. This is advantageous in that the sinkertrench is self-aligned to the active region. In this embodiment, thewidths of the sinker trench and the gate trenches and spacing S1 betweensinker trench 106 and the active region need to be carefully selectedtaking into account a number of factors. First, a ratio of width Ws ofsinker trench 106 to width Wg of gate trenches 112 needs to be selectedso that upon completion of the trench etch step sinker trench 106 andgate trenches 112 terminate at the desired depths. Second, the widthratio as well as spacing S1 needs to be carefully selected to minimizemicro-loading effect which occurs when trenches with different featuresare simultaneously etched. Micro-loading effect, if not addressedproperly, may cause trenches with a wide opening have a wider bottomthan top. This can lead to such problems as formation of pin-holes inthe conductive material in the sinker trench. The micro-loading effectcan also be minimized by selecting proper etch material. Third, thewidths of the trenches and spacing S1 impact the device on-resistanceRon. In the article by A. Andreini, et al., titled “A New IntegratedSilicon Gate Technology Combining Bipolar Linear, CMOS Logic, and DMOSPower Parts,” IEEE Transaction on Electron Devices, Vol. ED-33, No. 12,December, 1986, pp 2025-2030, a formula is set forth in section IV-B atpage 2028 which can be used to determine the optimum trench widths andspacing S1 for the desired Ron. Although the power device described inthis article uses a diffusion sinker, the same principles relating tooptimizing Ron can be applied in the present invention. This article isincorporated herein by reference.

The ratio of the width of the sinker trench to that of the gate trenchesis also dependent on the type of conductive material used in the sinkertrench. In general, a ratio of the sinker trench width to the gatetrench width of less than 10:1 is desirable. In one embodiment whereindoped polysilicon is used as the conductive material, a ratio of sinkertrench width to gate trench width of less than 5:1 is desirable. Forexample, for a gate trench width of 0.5 μm, a sinker trench width in therange of about 0.7 μm to 2.5 μm would be selected. If a metal or otherhighly conductive material is used in the sinker trench, a higher ratio(e.g., 3:1) is more desirable. Other than the relative width of thetrenches, spacing S1 between the sinker trench and the active regionalso impacts the micro-loading effect. A smaller spacing generallyresults in reduced micro-loading effect.

In one embodiment, the depth of the gate trenches in the epitaxial layeris selected to be close to the interface between substrate 102 andepitaxial layer 104 so that a slightly wider sinker trench would reachthrough to contact substrate 102. In an alternate embodiment, both thegate trenches and the sinker trench terminate within substrate 102.

In another embodiment, the sinker trench and the gate trenches areformed at different times. Thought the sinker trench would not beself-aligned to the active region, spacing S1 is not a criticaldimension. Advantages of forming the two trenches at different timesinclude elimination of the micro-loading effect, and the ability tooptimize each trench separately.

In accordance with an embodiment of the present invention, a method offorming the power transistor shown in FIG. 1 wherein the sinker trenchand gate trenches are formed simultaneously, is as follows. Epitaxiallayer 104 is formed over substrate 102. Next, a masking layer is used topattern the gate trench and sinker trench openings. Conventional plasmaetch techniques are used to etch the silicon to form the sinker trenchand gate trenches. An insulating layer, e.g., oxide, is then formedalong sidewalls and bottom of both the gate trenches and the sinkertrench. Increasing the insulating thickness or increase in thedielectric constant of the insulating material is advantageous inminimizing the area between the depletion region and sinker trench,distance S1, as some of the voltage from the depletion layer will besupported by the insulating layer thus reducing consumed silicon area byuse of a sinker trench.

A nitride layer is formed over the oxide layer in all trenches. Theoxide and nitride layers are then removed from the bottom of the sinkertrench using conventional photolithography and anisotropic etchtechniques thus leaving an oxide-nitride bi-layer along the sinkertrench sidewalls. Alternatively, a combination of anisotropic andisotropic etching or isotropic etching alone can be used. Thecombination of anisotropic and isotropic etching can advantageously beused to respectively remove the nitride and oxide layers from lowersidewall portions of the trench sinker (e.g., those lower sidewallportions extending in the substrate or even in the epitaxial layer—thiswould advantageously reduce the on-resistance). The resulting thickerbi-layer of dielectric along sinker trench sidewalls is advantageouslycapable of withstanding higher drain voltages. The sinker trench andgate trenches are then filled with in-situ doped polysilicon. The dopedpolysilicon is then etched back to planarize the top of the polysiliconin the trenches with the top surface of epitaxial layer 104. Next, usinga masking layer to cover the sinker trench, the polysilicon andoxide-nitride bi-layer are removed from the gate trenches. The gatetrenches are then lined with a gate oxide layer and filled with gatepolysilicon material. The excess gate polysilicon over the sinker trenchis removed using a conventional photolithography and etch process topattern the gate electrode. The remaining process steps for forming theinsulating layer over the gate electrodes, the well regions, the sourceregions, the source and drain metal contact layers, as well as othersteps to complete the device are carried out in accordance withconventional methods.

In an alternate method, after trenches are formed, a thick oxide layer(as mentioned above, to reduce the spacing of the sinker trench to thewell region) is formed along the sidewalls and bottom of the gate andsinker trenches. The thick oxide layer is then removed from the bottomof the sinker trenches using conventional photolithography andanisotropic etch techniques thus leaving the sidewalls of the sinkertrench lined with the thick oxide while the gate trenches are protected.Alternatively, a combination of anisotropic and isotropic etching can beused to also remove the thick oxide from lower portions of the trenchsinker sidewalls. The oxide layer may act as a sacrificial insulatinglayer for the gate trenches to improve the gate oxide integrity. Thesinker trench and gate trenches are then filled with in-situ dopedpolysilicon. The doped polysilicon is then etched back to planarize thetop of the polysilicon in the trenches with the top surface of epitaxiallayer 104. Next, using a masking layer to cover the sinker trench, thepolysilicon and insulating layer are removed from the gate trenches. Thegate trenches are then lined with a gate insulating layer and filledwith gate polysilicon material. The excess gate polysilicon over thesinker trenches is removed using a conventional photolithography andetch process to pattern the gate electrode. The remaining process stepsfor forming the insulating layer over the gate electrodes, the wellregions, the source regions, the source and drain metal contact layers,as well as other steps to complete the device are carried out inaccordance with conventional methods.

In another method, once trenches are formed, an insulating layer, e.g.,gate oxide, is formed (grown or deposited) along the sidewalls andbottom of the gate and sinker trenches. The gate oxide layer is thenremoved from the bottom of the sinker trenches using conventionalphotolithography and anisotropic etch techniques thus leaving an oxidelayer lining the sidewalls of the sinker trench while the gate trenchesare protected. Alternatively, a combination of anisotropic and isotropicetching or isotropic etching alone can be used. The combination ofanisotropic and isotropic etching can advantageously be used to removethe gate oxide layer from lower sidewall portions of the trench sinker(e.g., those lower sidewall portions extending in the substrate or evenin the epitaxial layer—this would advantageously reduce theon-resistance). The sinker trench and gate trenches are then filled within-situ doped polysilicon. The doped polysilicon is then patterned usingconventional photolithography techniques and etched to form both thesinker (drain) and gate electrodes. The remaining process steps forforming the insulating layer over the gate electrodes, the well regions,the source regions, the source and drain metal contact layers, as wellas other steps to complete the device are carried out in accordance withconventional methods.

In yet another method, the sinker trench and gate trenches are formedindependently by using separate masking steps. For example, using afirst set of masks and processing steps the gate trenches are definedand etched, lined with gate oxide, and filled with polysilicon. Using asecond set of masks and processing steps the sinker trench is definedand etched, lined with dielectric layer along its sidewalls, and filledwith a conductive material. The order in which the sinker trench andgate trenches are formed may be reversed.

FIG. 2 shows a simplified top layout view of the power device withsinker trench in accordance with an exemplary embodiment of theinvention. The FIG. 2 layout view depicts a stripe-shaped cellconfiguration. Stripe-shaped gate trenches 212 a extend vertically andterminate in horizontally-extending gate trenches 212 b. As shown, thethree groups of striped gate trenches are surrounded by a contiguoussinker trench 206. In an alternate embodiment shown in FIG. 3, sinkertrenches 306 are disposed between groups of gate trenches (only two ofwhich are shown) and are repeated at such frequency and spacing asdictated by the desired Ron. In one variation of this embodiment, toachieve the same Ron as the back-side drain contact approach, thespacing between adjacent sinker trenches needs to be two times thethickness of the wafer. For example, for a 4 mils thick wafer, thesinker trenches may be spaced from one another by approximately 8 mils.For even a lower Ron, the sinker trenches may be placed closer together.In yet another embodiment shown in FIG. 4, striped gate trenches 412extend horizontally, and vertically extending sinker trenches 406separate the different groups of gate trenches. Sinker trenches 406 areinterconnected by a metal interconnect 432. Metal interconnect is shownas being enlarged along the right side of the figure forming a drain padfor bond-wire connection. Also a gate pad 430 is shown in a cut-outcorner of one of the groups of gate trenches.

FIG. 5 shows a top view of a die housing the power device with sinkertrenches in accordance with an embodiment of the invention. The smallcircles depict the balls of a ball grid array package. The outerperimeter region 506 includes the sinker trench, and the balls in outerperiphery region 506 thus provide the drain connection. Central region507 represents the active region and the balls inside this regionprovide the source connection. The small square region 530 at the bottomleft corner of central region 508 represents the gate pad and the ballinside region 530 provides the gate connection.

As is readily apparent, the sinker trench structure 106 in FIG. 1 may beused to bring the backside connection of any power device to the topsurface and as such is not limited to use with vertical trenched-gatepower MOSFETs. Same or similar sinker trench structures may be similarlyintegrated with such other vertically conducting power devices as planargate MOSFETs (i.e., MOSFETs with the gate and its underlying channelregion extending over and parallel to the silicon surface), and powerdiodes to make the anode or cathode contact regions available along thetop for interconnection. Many other variations and alternatives arepossible, including use of shielded gate and dual gate structures indifferent combinations with various charge balancing techniques many ofwhich are described in detail in the above-referenced commonly assignedpatent application Ser. No. 11/026,276 titled “Power SemiconductorDevices and Methods of Manufacture” filed Dec. 29, 2004, which isincorporated herein by reference in its entirety. Also, although FIGS.2-5 show layout implementations based on the open cell configuration,the invention is not limited as such. The structure shown in FIG. 1 canalso be implemented in any one of a number of well known closed cellconfigurations. Lastly, the dimensions in the cross section view in FIG.1 and the top layout views in FIGS. 2-5 are not to scale and are merelyillustrative.

1. A semiconductor power device comprising: a plurality of groups ofstripe-shaped trenches extending in a silicon region over a substrate; acontiguous sinker trench completely surrounding each group of theplurality of stripe-shaped trenches so as to isolate the plurality ofgroups of stripe-shaped trenches from one another, the contiguous sinkertrench extending from a top surface of the silicon region through thesilicon region and terminating with in the substrate, the contiguoussinker trench being wider and extending deeper than the plurality ofstripe-shape trenches, the contiguous sinker trench being lined with aninsulator only along the sinker trench sidewalls so that a conductivematerial filling the contiguous sinker trench makes electrical contactwith the substrate along the bottom of the contiguous sinker trench andmakes electrical contact with an interconnect layer along the top of thecontiguous sinker trench.
 2. The semiconductor power device of claim 1wherein the silicon region is an epitaxial layer and the plurality ofstripe-shape trenches are gate trenches, the semiconductor devicefurther comprising: a well region of a second conductivity type in theepitaxial layer; source regions of the first conductivity type in thewell region, the source regions flanking the gate trenches; a gatedielectric layer lining at least the sidewalls of each gate trench; anda gate electrode at least partially filling each gate trench, wherein agate electrode contact layer electrically contacting the gateelectrodes, a source contact layer electrically contacting the sourceregions, and a drain contact layer electrically contacting the substrateare all along one surface of the semiconductor power device.
 3. Thesemiconductor power device of claim 1 wherein the conductive materialincludes one or more of doped polysilicon, selective epitaxial silicon(SEG), metal, and metallic compound.
 4. The semiconductor power deviceof claim 1 wherein the insulator comprises one of oxide, siliconnitride, silicon oxynitride, multilayer of oxide and nitride, a low kinsulating material, and a high k insulating material.
 5. Thesemiconductor power device of claim 1 wherein two of the plurality ofgroups of strip-shaped trenches include different number of strip-shapedtrenches.
 6. The semiconductor power device of claim 1 furthercomprising a back-side metal layer extending along a bottom surface ofthe substrate.
 7. The semiconductor power device of claim 1 wherein theconductive material comprises polysilicon and a ratio of a width of thesinker trench to a width of each gate trench is less than five to one.8. The semiconductor power device of claim 1 wherein the conductivematerial comprises a metal and a ratio of a width of the sinker trenchto a width of each gate trench is less than three to one.
 9. Thesemiconductor power device of claim 2 wherein the dielectric layerlining the sidewalls of the contiguous sinker trench is thicker than thegate dielectric layer.
 10. The semiconductor power device of claim 2wherein the dielectric layer lining the sidewalls of the contiguoussinker trench and the gate dielectric layer have the same thickness. 11.The semiconductor power device of claim 2 wherein each gate trenchfurther includes a thick bottom dielectric below the gate electrode, thethick bottom dielectric having a greater thickness than that of the gatedielectric.
 12. The semiconductor power device of claim 1 wherein eachgate trench includes a shield electrode below the gate electrode, thegate and shield electrodes being insulated from one another.